Ion symporters are membrane proteins that mediate the co-transport of two different ion species across themembrane. They play important roles in human physiology; the Na+/i- symporter, for example, is responsible for accumulation of I- in the thyroid gland. Despite their importance a detailed mechanistic understanding of how symporters work is lacking. The long term objective of this proposal is the development of a detailed molecular model for the co-transport oftwo ions, Na+ and K+ by KtrAB, a bacterial symporter. The major aims of this proposal are: 1 - Characterize the transport properties of the KtrAB symporter. 2 - Characterize the structure and function of KtrA. 3 - Determine the crystal structure of the membrane protein KtrB.